[netemul] Basic layout for netemul runner/sandbox

- Netemul runner offers fuchsia.sys.Runner and calls sandbox to start
tests in tailored environment for network emulation
- Netemul sandbox is the "root process" that will create the emulated
environment, launch the tests and observe exit codes

TESTS: no significant tests here apart from a simple test to make sure
runner and sandbox are creating processes as intended

Change-Id: Id2b8f99528a7ede3c585e15e816f756a621f672e
25 files changed